In recent years, particulates, contained in exhaust gases that are discharged from internal combustion engines of vehicles such as buses and trucks and construction equipment, have raised serious problems since those particulates are harmful to the environment and the human body.
For this reason, various honeycomb structural bodies, made from porous ceramics, have been proposed as filters capable of collecting particulates from exhaust gases to purify the exhaust gases.
Conventionally, with respect to the above-mentioned honeycomb structural body, a columnar honeycomb structural body 30 in which, as shown in FIG. 6, a number of through holes 31 are placed in parallel with one another in the length direction with partition wall 33 interposed therebetween has been known. As shown in FIG. 6(b), the through hole 31 is sealed with a sealing material 32 at either of ends of its exhaust gas inlet side or exhaust gas outlet side, so that exhaust gases that have entered one through hole 31 are discharged from another through hole 31 after having always passed through the partition wall 33 that separates the through holes 31. In other words, when the honeycomb structural body 30 is installed in an exhaust gas passage of an internal combustion engine, particulates in exhaust gases discharged from the internal combustion engine are captured by the partition wall 33 when passing through the honeycomb structural body 30, so that the exhaust gases are purified.
Moreover, with respect to such a honeycomb structural body, the following structure has been proposed: a through hole with the end on the exhaust gas outlet side being sealed (hereinafter, also referred to as inlet-side through hole) is formed as a through hole with a larger capacity (hereinafter, also referred to as large-capacity through hole) and a through hole with the end on the exhaust gas inlet side being sealed (hereinafter, also referred to as outlet-side through hole) is formed as a through hole with a smaller capacity (hereinafter, also referred to as small-capacity through hole), so that the aperture ratio on the exhaust gas inlet side is made relatively greater than the aperture ratio on the exhaust gas outlet side.
JP Kokai Sho 56-124418 has disclosed a ceramic filter in which through holes having shapes, such as a triangle, a hexagonal shape, a circular shape and a swelled shape, are formed. Moreover, U.S. Pat. No. 4,276,071 (FIGS. 5a and 5p), JP Kokai Sho 56-124417, JP Kokai Sho 62-96717 and U.S. Pat. No. 4,364,761 (FIGS. 5a to 5p) have disclosed arrangements similar to that of JP Kokai Sho 56-124418.
Microfilms of Japanese Utility Model Application No. 56-187890 (JU Kokai Sho 58-92409 (FIG. 6, page 4) have disclosed an exhaust gas filter in which triangular through holes and hexagonal through holes are formed with cell pitches of large-capacity through holes being set approximately in a range from 1.0 to 2.5 mm.
U.S. Pat. No. 4,416,676 (FIGS. 1 to 4) has disclosed a honeycomb filter in which through holes having shapes, such as a triangle, a square, an octagonal shape and a round shape, are formed while the relationship between: the wall thickness between large-capacity through holes; and the wall thickness between the large-capacity through hole and the small-capacity through hole; being defined.
JP Kokai Sho 58-196820, JP Kokoku Hei 3-49608 and U.S. Pat. No. 4,417,908 (FIGS. 3 to 17) have disclosed honeycomb filters in which through holes having shapes such as a triangle, a square and a hexagonal shape as well as honeycomb filters in which the number of through holes on the inlet side is made greater than the number of through holes on the outlet side so that the aperture rate on the exhaust gas inlet side is made relatively greater than the aperture rate on the exhaust gas outlet side.
U.S. Pat. No. 4,420,316 (FIGS. 6 to 9) has disclosed a honeycomb filter in which the number of sealed through holes is modified, which relates to a technique for improving the gas flow rate in the wall portions.
JP Kokai Sho 58-150015 has disclosed a filter which is provided with square through holes and rectangular through holes, with the cross-sectional shape of the through holes being formed into a tapered shape so as to be changed from the gas inlet side toward the outlet side.
JP Kokai Hei 5-68828 and the Japanese Patent gazette No. 3130587 (page 1) have disclosed honeycomb filters in which triangular through holes and hexagonal through holes are formed and the capacity rate of the large-capacity through holes is set to 60 to 70% while the capacity rate of the small-capacity through holes is set to 20 to 30%, with the cell pitch of the large-capacity through holes being set to approximately in a range from 2.5 to 5.0 mm.
French Patent No. 2789327 has disclosed a filter that is provided with through holes having shapes such as a rectangular shape, a square shape, a hexagonal shape and an octagonal shape, with the cross-sectional shape of the through holes being formed into a tapered shape so as to be changed from the gas inlet side toward the outlet side.
International Publication No. 02/100514 and JP Kokai 2001-334114 (FIG. 2) have disclosed filters in which through holes having a round shape and a hexagonal shape are formed. These have also disclosed filter elements in which the ratio of the total area of the cross-section of small-capacity through holes to the total area of the cross-section of large-capacity through holes is set in a range from 40 to 120%.
International Publication No. 02/10562 has disclosed a filter in which square through holes and hexagonal through holes are formed, with the ratio of cross-sections thereof being set in a range from 3:1 to 4:1.
International Publication No. 03/20407 has disclosed a honeycomb structural body in which square through holes are formed with a varied ratio of cross-sectional areas.
In the honeycomb structural bodies described in these patent documents, since the aperture ratio on the exhaust gas inlet side is made relatively greater in comparison with the honeycomb structural body in which the aperture ratio on the exhaust gas inlet side and the aperture ratio on the exhaust gas outlet side are equal to each other, it becomes possible to increase the limiting collection amount of particulates, to lengthen the period up to the recovery process and to miniaturize the structure, when used as a filter for purifying exhaust gases.
However, it has been found that, although these honeycomb structural bodies slightly reduce the rate of increase in pressure loss upon collection of particulates in comparison with a honeycomb structural body in which the aperture ratio on the exhaust gas inlet side and the aperture ratio on the exhaust gas outlet side are equal to each other, they already have high pressure loss even in a state having collected no particulates before the start of use, and consequently have high pressure loss over the entire period of use.
Moreover, the flow rate of exhaust gases is affected not only by the relationship between the displacement of an internal combustion engine that discharges exhaust gases and honeycomb structural body, but also by the operation condition of the internal combustion engine. For example, in the case of automobiles, the flow rate of exhaust gases discharged from the internal combustion engine fluctuates every moment in response to the driving modes (such as, flat-way driving, slope-way driving, high speed driving and low speed driving), and when the flow rate of exhaust gases increases, the back pressure caused by the honeycomb structural body becomes higher, resulting in an abrupt rise in pressure loss. In such cases, since a load is imposed on an engine, the riding comfort of the automobile deteriorates, resulting in a problem of giving the discomfort to the driver.